Abstract

The Paraná-Etendeka igneous province is one of the largest flood volcanic provinces in the world; peak magmatic activity at 132 Ma is believed to have occurred about 5 m.y. before the birth of south Atlantic sea floor and development of rift basins along the Brazilian coastal margin. Anisotropy of magnetic susceptibility (AMS) measurements on 283 samples (28 flows and 3 sills) from the Etendeka igneous province of Namibia and 180 samples (21 flows) from the Paraná province in Brazil reveal remarkably consistent fabric orientations with maximum susceptibility (K1) axes subhorizontal and parallel to the rifted margin. The AMS results are most likely due to shape anisotropy reflecting magma flow directions, suggesting that lava flows and intrusive conduits near the eventual rifted margin were controlled by structures having topographic expression in existence at the time of peak flood volcanism. These results imply that rifting preceded flood volcanism, at least in the portion of the magmatic province within 100 km of the nascent Mid-Atlantic Ridge.